Fabrication and characterization of Ni/ScSZ cermet anodes for IT-SOFCs

In this study the fabrication and characterization of Ni/10ScSZ (Ni/10 mol% Sc2O3–90 mol% ZrO2) and Ni/10Sc1CeSZ (Ni/10 mol% Sc2O3–1 mol% CeO2–89 mol% ZrO2) cermet anode films was studied and compared. Both 10ScSZ and 10Sc1CeSZ electrolyte powders showed tetragonal and cubic phases at room temperature, respectively. The NiO/10ScSZ and NiO/10Sc1CeSZ composites with 10–60 vol% of Ni content were prepared by mixing as-received commercial powders of NiO, 10ScSZ and 10Sc1CeSZ followed by ink preparation. Samples were sintered for 1 h at temperatures of 1250–1350 °C. All the cermet films were then reduced under a mixture of hydrogen (10%) and nitrogen (90%) at 800 °C for 2 h. The effect of Ni content and sinter temperature on the DC electrical conductivity were investigated, and the results showed a sharp change in conductivity at around 30 vol% Ni, corresponding to continuity/discontinuity of the Ni–Ni contact network, and the conductivity increased as the sinter temperature increased from 1250 to 1350 °C. An acceptable electrical conductivity at 700 °C for these cermet films was obtained at >40 vol% Ni, consistent with behaviour reported for more conventional Ni/YSZ cermets. The effect of sinter temperature on the microstructure and porosity of Ni/10Sc1CeSZ and Ni/10ScSZ cermet films was also investigated. This revealed that the porosity of the cermet films with the same Ni content decreased as the sinter temperature increased and that, for a given sinter temperature, the porosity of the cermet films increased with Ni content. The porosities of 40Ni/60ScCeSZ (40 vol% Ni/60 vol% 10Sc1CeSZ) and 40Ni/60ScSZ (40 vol% Ni/60 vol% 10ScSZ) anodes sintered at 1250, 1300 and 1350 °C for 1 h were in the range of 30–45%. Electrochemical measurement of symmetrical cells using an 8YSZ electrolyte at 700 °C revealed that the lowest electrode polarization resistance of 40Ni/60ScCeSZ and 40Ni/60ScSZ anodes was obtained at sinter temperatures of 1350 °C and 1300 °C respectively. Carbon deposition over 40Ni/60ScCeSZ, 40Ni/60ScSZ and 40Ni/60YSZ catalysts was evaluated at 700 °C for 1 h at S/C = 0.8 and the results showed that the ratio of deposited carbon was lower in the case of Ni/10ScSZ and Ni/10Sc1CeSZ compared to Ni/YSZ (0.35). Overall, Ni/10Sc1CeSZ and Ni/10ScSZ cermets having 40 vol% Ni were found to be optimum, with the 40Ni/60ScCeSZ cermet proving to be better than 40Ni/60ScSZ cermet in terms of both electronic conductivity and electrode polarization resistance, with both materials exhibiting improved tolerance towards carbon deposition compared to Ni/YSZ.